224 



DISCOVERY REPORTS 



ridge with a maximum depth of water of under 2000 m . and an average depth of approxi- 

 mately 1300 m. Two further Hnes of echo soundings were taken to the north of the ridge 

 by the 'Discovery II' during the season 193 1-2, and these have been of considerable 

 value in determining the 4000 m. contour. This contour is of great importance in this 

 area, as it marks the return, on the northern side of the ridge, to the average depth of 

 this part of the South Atlantic. 



The depth of water on the ridge gradually increases from South Georgia towards the 

 Shag Rocks till it reaches a maximum of 1650 m. about 55 miles from Willis Island. The 

 depth for the next 35 miles shows a certain amount of variation from the average, a 

 sounding of 997 m . occurring at a point a few miles due south of the ' William Scoresby 's ' 

 original depth of 232 m. Farther on the bottom rises sharply from 1236 to 459 m. in less 

 than 4 miles, and for the remaining distance to the Shag Rocks the average depth of 

 water is about 150 m. 



The shelf surrounding the Shag Rocks appears to extend for a considerable distance. 

 Thirty miles to the south the soundings at present available show a depth of iioo m., 

 and in a northerly direction the depth only reaches 2000 m. some 35 miles from the 

 Rocks. 



SOUTH GEORGIA TO THE SOUTH SANDWICH ISLANDS 

 (Plates XLV, XLVI, Charts 3, 4, 5) 



The connexion between South Georgia and the Gierke Rocks, which lie about 40 

 miles east-south-east of the south-eastern point of South Georgia, is quite definitely 

 shown by the existing soundings, the average depth of water between the Rocks and 

 South Georgia being from 100 to 130 m. (Chart 5). According to Holtedahl,i the Gierke 

 Rocks are of considerable geological interest, as one of the specimens which he obtained 

 from there in 1928, and which is reported to occur as veins in the granite, is said to 

 represent a basic dyke. A detailed examination of this specimen has yet to be made. In 

 conclusion, Holtedahl says of the Gierke Rocks that "the find of an originally deep- 

 seated rock, cut by dykes, not very far from the intrusive masses of South Georgia, and 

 in the direction towards the volcanic ridge of the South Sandwich Islands is of consider- 

 able interest". 



From the Gierke Rocks to the South Sandwich Islands, a distance of about 280 miles, 

 the connecting ridge is not very clearly defined at the moment. There is, however, 

 sufficient evidence from the soundings to show that a connexion can be traced. Fig. 2 a, 

 which is based on a line of echo soundings from north to south across the ridge, and 

 Fig. 4 a, which is a section crossing the ridge farther to the east and at a difi"erent angle, 

 both show that there is a gradual rise from the bottom of the Scotia Sea basin, followed 

 by a sharp drop into the north-western end of the South Sandwich Deep. The rise on 

 the southern side varies between 400 and 700 m. and the depth of water on the ridge 

 averages about 2700 m. The minimum sounding found was 1935 m. in approximately 



' Holtedahl, loc. cif., supra, p. 102. 



